Lateral Heterostructures of Graphene and h-BN with Atomic Lattice Coherence and Tunable Rotational Order.

In-plane heterostructures of graphene and hexagonal boron nitride (h-BN) exhibit exceptional properties, which are highly sensitive to the structure of the alternating domains. Nevertheless, achieving accurate control over their structural properties, while keeping a high perfection at the graphene-h-BN boundaries, still remains a challenge. Here, the growth of lateral heterostructures of graphene and h-BN on Rh(110) surfaces is reported. The choice of the 2D material, grown firstly, determines the structural properties of the whole heterostructure layer, allowing to have control over the rotational order of the domains. The atomic-scale observation of the boundaries demonstrates a perfect lateral matching. In-plane heterostructures floating over an oxygen layer have been successfully obtained, enabling to observe intervalley scattering processes in graphene regions. The high tuning capabilities of these heterostructures, along with their good structural quality, even around the boundaries, suggest their usage as test beds for fundamental studies aiming at the development of novel nanomaterials with tailored properties.

C60 self-orientation on hexagonal boron nitride induced by intermolecular coupling.

A deep grasp of the properties of the interface between organic molecules and hexagonal boron nitride (h-BN) is essential for the full implementation of these two building blocks in the next generation of electronic devices. Here, using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS), we report on the geometric and electronic features of C60 evaporated on a single layer of h-BN grown on a Rh(110) surface under ultra-high vacuum. Two different molecular assemblies of C60 on the h-BN/Rh(110) surface were observed. The first STM study at room temperature (RT) and at low temperatures (40 K) looked at the molecular orientation of C60 on a two-dimensional layered material. Intramolecular-resolution images demonstrate the existence of a phase transition of C60 over the h-BN/Rh(110) surface similar to that found on bulk solid C60. At RT molecules exhibit random orientations, while at 40 K such rotational disorder vanishes and they adopt a common orientation over the h-BN/Rh(110) surface. The decrease in thermal energy allows recognition between C60 molecules, and they become equally oriented in the configuration at which the van der Waals intermolecular interactions are optimized. Bias-dependent submolecular features obtained by means of high-resolution STM images are interpreted as the highest occupied and lowest unoccupied molecular orbitals. STS data showed that fullerenes are electronically decoupled from the substrate, with a negligible charge transfer effect if any. Finally, the very early stages of multilayer growth were also investigated.

From Permeation to Cluster Arrays: Graphene on Ir(111) Exposed to Carbon Vapor.

Our scanning tunneling microscopy and X-ray photoelectron spectroscopy experiments along with first-principles calculations uncover the rich phenomenology and enable a coherent understanding of carbon vapor interaction with graphene on Ir(111). At high temperatures, carbon vapor not only permeates to the metal surface but also densifies the graphene cover. Thereby, in addition to underlayer graphene growth, upon cool down also severe wrinkling of the densified graphene cover is observed. In contrast, at low temperatures the adsorbed carbon largely remains on top and self-organizes into a regular array of fullerene-like, thermally highly stable clusters that are covalently bonded to the underlying graphene sheet. Thus, a new type of predominantly sp2-hybridized nanostructured and ultrathin carbon material emerges, which may be useful to encage or stably bind metal in finely dispersed form.

Tuning the van der Waals Interaction of Graphene with Molecules via Doping.

We use scanning tunneling microscopy to visualize and thermal desorption spectroscopy to quantitatively measure that the binding of naphthalene molecules to graphene, a case of pure van der Waals interaction, strengthens with n and weakens with p doping of graphene. Density-functional theory calculations that include the van der Waals interaction in a seamless, ab initio way accurately reproduce the observed trend in binding energies. Based on a model calculation, we propose that the van der Waals interaction is modified by changing the spatial extent of graphene's π orbitals via doping.

Growth of 1D ClAlPc molecular chains mediated by graphene moiré patterns.

The on-surface formation of iso-oriented 1D molecular architectures, with high structural perfection, on 2D materials has been a long-sought objective. However, such realization has been troublesome and limited, and it still remains an experimental challenge. Here, the quasi-1D stripe-like moiré pattern, arising at the interface of graphene grown on Rh(110), has been used to guide the formation of 1D molecular wires of π-conjugated, non-planar, chloro-aluminum phthalocyanine (ClAlPc) molecules, brought together by van der Waals interactions. Using scanning tunnelling microscopy (STM) under ultra-high vacuum (UHV) at 40 K, the preferential adsorption orientations of the molecules at low coverages have been investigated. The results shed light on the potential signature of graphene lattice symmetry breaking, induced by the incommensurate quasi-1D moiré pattern of Gr/Rh(110), as the subtle mechanism behind this templated growth of 1D molecular structures. For coverages close to 1 ML, the molecule-molecule interactions favor a closely packed square lattice arrangement. The present work provides new insights to tailor 1D molecular structures on graphene grown on a non-hexagonal metal substrate.

Aperiodic Modulation of Graphene Driven by Oxygen-Induced Reconstruction of Rh(110).

Artificial nanostructuring of graphene has served as a platform to induce variations in its structural and electronic properties, fostering the experimental observation of a wide and fascinating phenomenology. Here, we present an approach to graphene tuning, based on Rh(110) surface reconstruction induced by oxygen atoms intercalation. The resulting nanostructured graphene has been characterized by scanning tunneling microscopy (STM) complemented by low-energy electron microscopy (LEEM), micro low-energy electron diffraction (µ-LEED), micro angle-resolved photoemission spectroscopy (µ-ARPES), and micro X-ray photoelectron spectroscopy (µ-XPS) measurements under ultrahigh vacuum (UHV) conditions at room temperature (RT). It is found that by fine-tuning the O2 exposure amount, a mixture of missing row surface reconstructions of the metal surface below the graphene layer can be induced. This atomic rearrangement under the graphene layer results in aperiodic patterning of the two-dimensional (2D) material. The electronic structure of the resulting nanostructured graphene is dominated by a linear dispersion of the Dirac quasiparticles, characteristic of its free-standing state but with a p-doping character. The local effects of the underlying missing rows on the interfacial chemistry and on the quasiparticle scattering processes in graphene are studied using atomically resolved STM images. The possibilities offered by this nanostructuring approach, which consists in inducing surface reconstructions under graphene, could provide a novel tuning strategy for this 2D material.

MoS2 Photoelectrodes for Hydrogen Production: Tuning the S-Vacancy Content in Highly Homogeneous Ultrathin Nanocrystals.

Tuning the electrocatalytic properties of MoS2 layers can be achieved through different paths, such as reducing their thickness, creating edges in the MoS2 flakes, and introducing S-vacancies. We combine these three approaches by growing MoS2 electrodes by using a special salt-assisted chemical vapor deposition (CVD) method. This procedure allows the growth of ultrathin MoS2 nanocrystals (1-3 layers thick and a few nanometers wide), as evidenced by atomic force microscopy and scanning tunneling microscopy. This morphology of the MoS2 layers at the nanoscale induces some specific features in the Raman and photoluminescence spectra compared to exfoliated or microcrystalline MoS2 layers. Moreover, the S-vacancy content in the layers can be tuned during CVD growth by using Ar/H2 mixtures as a carrier gas. Detailed optical microtransmittance and microreflectance spectroscopies, micro-Raman, and X-ray photoelectron spectroscopy measurements with sub-millimeter spatial resolution show that the obtained samples present an excellent homogeneity over areas in the cm2 range. The electrochemical and photoelectrochemical properties of these MoS2 layers were investigated using electrodes with relatively large areas (0.8 cm2). The prepared MoS2 cathodes show outstanding Faradaic efficiencies as well as long-term stability in acidic solutions. In addition, we demonstrate that there is an optimal number of S-vacancies to improve the electrochemical and photoelectrochemical performances of MoS2.

Ethylene irradiation: a new route to grow graphene on low reactivity metals.

A novel technique for growing graphene on relatively inert metals, consisting in the thermal decomposition of low energy ethylene ions irradiated on hot metal surfaces in ultrahigh vacuum, is reported. By this route, we have grown graphene monolayers on Cu(111) and, for the first time, on Au(111) surfaces. For both noble metal substrates, but particularly for Au(111), our scanning tunneling microscopy and spectroscopy measurements provide sound evidence of a very weak graphene-metal interaction.

Observation of Superconducting Collective Modes from Competing Pairing Instabilities in Single-Layer NbSe2.

In certain unconventional superconductors with sizable electronic correlations, the availability of closely competing pairing channels leads to characteristic soft collective fluctuations of the order parameters, which leave fingerprints in many observables and allow the phase competition to be scrutinized. Superconducting layered materials, where electron-electron interactions are enhanced with decreasing thickness, are promising candidates to display these correlation effects. In this work, the existence of a soft collective mode in single-layer NbSe2 , observed as a characteristic resonance excitation in high-resolution tunneling spectra is reported. This resonance is observed along with higher harmonics, its frequency Ω/2Δ is anticorrelated with the local superconducting gap Δ, and its amplitude gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (TC and HC2 ), which sets an unambiguous link to the superconducting state. Aided by a microscopic model that captures the main experimental observations, this resonance is interpreted as a collective Leggett mode that represents the fluctuation toward a proximate f-wave triplet state, due to subleading attraction in the triplet channel. These findings demonstrate the fundamental role of correlations in superconducting 2D transition metal dichalcogenides, opening a path toward unconventional superconductivity in simple, scalable, and transferable 2D superconductors.

Proximity Effects on the Charge Density Wave Order and Superconductivity in Single-Layer NbSe2.

Collective electronic states such as the charge density wave (CDW) order and superconductivity (SC) respond sensitively to external perturbations. Such sensitivity is dramatically enhanced in two dimensions (2D), where 2D materials hosting such electronic states are largely exposed to the environment. In this regard, the ineludible presence of supporting substrates triggers various proximity effects on 2D materials that may ultimately compromise the stability and properties of the electronic ground state. In this work, we investigate the impact of proximity effects on the CDW and superconducting states in single-layer (SL) NbSe2 on four substrates of diverse nature, namely, bilayer graphene (BLG), SL-boron nitride (h-BN), Au(111), and bulk WSe2. By combining low-temperature (340 mK) scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we compare the electronic structure of this prototypical 2D superconductor on each substrate. We find that, even when the electronic band structure of SL-NbSe2 remains largely unaffected by the substrate except when placed on Au(111), where a charge transfer occurs, both the CDW and SC show disparate behaviors. On the insulating h-BN/Ir(111) substrate and the metallic BLG/SiC(0001) substrate, both the 3 × 3 CDW and superconducting phases persist in SL-NbSe2 with very similar properties, which reveals the negligible impact of graphene on these electronic phases. In contrast, these collective electronic phases are severely weakened and even absent on the bulk insulating WSe2 substrate and the metallic single-crystal Au(111) substrate. Our results provide valuable insights into the fragile stability of such electronic ground states in 2D materials.

Pseudo-ordered distribution of Ir nanocrystals on h-BN.

A 2D material consisting of a pseudo-ordered distribution of Ir nanocrystals supported on a h-BN/Rh(111) surface is presented here. The particular spatial distribution of the Ir nanoparticles is achieved thanks to the existence of a large variety of adsorption positions within the pores of the h-BN/Rh(111) nanomesh template with hexagonal symmetry. The resulting deviations of nanoparticle positions with respect to a perfect hexagonal lattice, which make this material of special interest in the field of optics, can be tuned by the temperature and the amount of Ir. Upon annealing, this material undergoes slight structural changes in the temperature range of 370-570 K and much more drastic ones, due to cluster coalescence, between 670 and 770 K. This relatively high onset of coalescence is encouraging for using this 2D material as a catalyst for reactions such as the oxidation of carbon monoxide or of nitrogen monoxide, which are especially relevant in the field of environmental science. Finally, metal nanostructures exhibiting regular geometries have been created from this material using a scanning tunneling microscope tip. Because of the insulating character of h-BN, these nanostructures could be very promising to use in the design of conductive nanotracks.

Graphene Tunable Transparency to Tunneling Electrons: A Direct Tool To Measure the Local Coupling.

The local interaction between graphene and a host substrate strongly determines the actual properties of the graphene layer. Here we show that scanning tunneling microscopy (STM) can selectively help to visualize either the graphene layer or the substrate underneath, or even both at the same time, providing a comprehensive picture of this coupling with atomic precision and high energy resolution. We demonstrate this for graphene on Cu(111). Our spectroscopic data show that, in the vicinity of the Fermi level, graphene π bands are well preserved presenting a small n-doping induced by Cu(111) surface state electrons. Such results are corroborated by Angle-Resolved Photoemission Spectra (ARPES) and Density Functional Theory with van der Waals (DFT + vdW) calculations. Graphene tunable transparency also allows the investigation of the interaction between the substrate and foreign species (such as atomic H or C vacancies) on the graphene layer. Our calculations explain graphene tunable transparency in terms of the rather different decay lengths of the graphene Dirac π states and the metal surface state, suggesting that it should apply to a good number of graphene/substrate systems.

Structure and Growth of Hexagonal Boron Nitride on Ir(111).

Using the X-ray standing wave method, scanning tunneling microscopy, low energy electron diffraction, and density functional theory, we precisely determine the lateral and vertical structure of hexagonal boron nitride on Ir(111). The moiré superstructure leads to a periodic arrangement of strongly chemisorbed valleys in an otherwise rather flat, weakly physisorbed plane. The best commensurate approximation of the moiré unit cell is (12 × 12) boron nitride cells resting on (11 × 11) substrate cells, which is at variance with several earlier studies. We uncover the existence of two fundamentally different mechanisms of layer formation for hexagonal boron nitride, namely, nucleation and growth as opposed to network formation without nucleation. The different pathways are linked to different distributions of rotational domains, and the latter enables selection of a single orientation only.